Method and apparatus for measuring neutron characteristics of a material surrounding a well bore

ABSTRACT

In the particular embodiments of the invention described herein, the formation surrounding a well bore is irradiated with a burst of neutrons and the neutron concentration is observed during selected time intervals after irradiation to determine the thermal neutron decay time characterizing the formation. In one embodiment, the neutron concentration is observed during a first interval one decay time long and during a second and subsequent interval two decay times long which starts immediately after the first interval. In another embodiment the two intervals are spaced by one decay time.

United States Patent 111 E Re. 28,477

Nelligan [45] Reissued July 8, 1975 [54] METHOD AND APPARATUS FOR3,155,901 11/1964 Hanken 324/61 MEASURING NEUTRON 3,164,720 1/1965Armistead 250/270 3,358,142 12/1967 Hopkinson et a1. 250/262CHARACTERISTICS OF A MATERIAL 3,373,280 3/1968 Mills, Jr. 250/262SURROUNDING A WELL BORE 3,379,882 4/1968 Youmansm, 250/262 [75]Inventor: William B. Nelligan, Danbury, 31379384 4/1968 YOUmFmS 250/262Conn. 3,402,294 9/1968 Bargamer, Jr. 250/261 [731 Assign: schlumbefgerTechnology Primary Examiner.lames W. Lawrence Corporanon New YorkAssistant Examiner-Davis L. Willis [22] Filed; Feb, 23, 1973 Attorney,Agent, or Firm-Brumbaugh, Graves,

21 Appl. No.2 335,165

Related US. Patent Documents VAIIAILE OlClLL Donohue & Raymond [57]ABSTRACT In the particular embodiments of the invention describedherein, the formation surrounding a well bore is irradiated with a burstof neutrons and the neutron concentration is observed during selectedtime intervals after irradiation to determine the thermal neutron decaytime characterizing the formation. In one embodiment, the neutronconcentration is observed during a first interval one decay time longand during a second and subsequent interval two decay times long whichstarts immediately after the first interval. In another embodiment thetwo intervals are spaced by one decay time.

67 Claims, 7 Drawing Figures Reissuecl July 8, 1975 Re. 28,477

4 Sheets-Sheet 1 FIG LU I05. 2: I o4- 2 l2 03. 5 H 8 0L [0 0 IO v I v ITIME AFTER IRRADIATION (p586) /6 /6E 0 I62 5 0L I6 I l j 3 I52 0 I45 2 0I42 /5 g I4 2 I42 8 Q IO I TIME AFTER |RRAD|AT|0N(AJSOC) IN\ ENIUR.WILLIAM B. NELLIGAN his ATTORNEYS Reissued July 8, 1975 COUNTING RATECOUNTING RATE 4 Sheets-Sheet 5 T|ME uN UNITS OF DECAY TIME T) P e r HWILLIAM B. NELLIGAN hil ATTORNEYS TIME-- (IN UNITS OF DECAY TIME 7.)

ReissueclJuly 8, 1975 28,477

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LOGIC GATE NETWORK T I I VARIABLE FLIP FLIP FLIP FLIP FLIP OSCILLATOR(TO PERIOD) f LgP FL ')P FL?P FL?P 40 37 4; 44 46 W 46 mo 47 we COUNTER5A 56 [..-CLOCK 4 1 w r I Q A I 3! FIG. 7- i4 3g 1 35 I\\\\ AWN I\SOURCE I IVWiV/UR Z3 WILLIAM B. NELLIGAN m I 5 e I e ,ywi 0M ATTORNEYS1 METHOD AND APPARATUS FOR MEASURING NEUTRON CHARACTERISTICS OF AMATERIAL SURROUNDING A WELL BORE Matter enclosed in heavy bracketsappears in the original patent but forms no part of this reissuespecification; matter printed in italics indicates the additions made byreissue.

This invention relates to measurement of the neutron characteristic timeconstants of an unknown material such as the decay time of thermalneutrons therein and, more particularly, to a new and improved methodand apparatus for measuring neutron characteristic time constants moreaccurately and rapidly.

One procedure for determining the character of unknown materials, suchas the earth formations through which a well bore passes, comprisesirradiating the material with neutrons for a selected period of time andthen determining the concentration of neutrons in the material atselected times after irradiation so that a neutron characteristic timeconstant of the material maybe ascertained. Because the various elementscapture thermal neutrons at different rates, the change of thermalneutron concentrations with time following irradiation will be differentfor materials containing different elements so that a determination ofthe rate of capture can be used to give an indication of the type ofmaterial irradiated. Usually, this neutron characteristic of thematerial is expressed as the thermal neutron decay time, which is thetime required for the thermal neutron concentration to decrease by afactor equal to the natural logarithm base e which is 2.718. in anotherprocedure the characteristic of the material known as the neutronslowing down time is determined by measuring the concentration of higherenergy neutrons at various times after irradiation.

When the earth formation material adjacent to a well bore is beinganalyzed, the variation of neutron concentration with time during theperiod immediately following irradiation is influenced to a large extentby material in the well bore. As the thermal neutron intensity reaches alow level, moreover, measurements are altered by background and noiseeffects. Consequently, there is only a certain period of time duringwhich the neutron characteristic time constants such as the thermalneutron decay time of the formation material can be determinedaccurately. The rate of decay of thermal neutrons with time and theslowing down time of higher energy neutrons vary widely for differentformation materials, however, so that the proper time interval followingneutron irradiation for characteristic time constant measurements is notthe same for different formations and, heretofore, it has been necessaryto measure the neutron concentration at many different times afterneutron irradiation in order to obtain an accurate de termination of aneutron characteristic time constant.

Accordingly, it is an object of the present invention to provide a newand improved method for measuring neutron characteristic time constantswhich overcomes the above-mentioned disadvantages of present methods.

Another object of the invention is to provide a new and improvedapparatus giving immediate and accurate indications of the thermalneutron decay time of a material being analyzed.

These and other objects of the invention are attained by measuring therate of change of the neutron intensity so as to indicate the neutroncharacteristic time constant of the material at a time after neutronirradiation which is dependent upon the characteristic time constant ofthe material. In this way, thermal neutron decay time measurements, forexample, can be made at the proper time for all materials regardless ofwhether the thermal neutron concentration decreases rapidly or slowly.Moreover, the durations of the time intervals during which the neutroncharacteristic time constant measurements are made are also varied inaccordance with the time constant of the materials so that the ratio ofthe neutron counting rates is a predetermined number when the timeintervals are properly selected. For optimum operation, the duration ofthe neutron irradiation intervals are also proportional to the timeconstant.

In a particular embodiment for thermal neutron decay time measurementsutilizing continuously repetitive bursts of neutron irradiation spacedat intervals about nine decay times in length, a first thermal neutroncount is taken during an interval one decay time long which begins twodecay times after the irradiation has stopped and a second thermalneutron count is taken during a second interval immediately after thefirst interval which is two decay times long. Preferably, the thermalneutron intensity may be measured by a detector which detects thecapture gamma rays produced in the formation. With this type ofdetector, it is preferable to take a background count during an intervalbeginning at least seven decay times after the neutron irradiation hasstopped. This background count is subtracted from both of the first andsecond interval counts in proportion to their duration. In anotherembodiment, improved results are obtained by spacing the first andsecond time intervals by one decay period. In this case, every fourthneutron burst is omitted and the background is measured only during aperiod at least eight decay times after the third burst, the threeneutron bursts being two decay times I: along long and occurring atintervals of eight decay times.

In the first embodiment, when the adjacent time in tervals are properlyset at one and two decay times, respectively, the ratio of the countingrate in the first interval to the counting rate in the second intervalis a fixed number 1.99. Consequently, the time intervals are adjusted ina two to one ratio until the ratio of counting rates equals 1.99 and thedelay between irradiation and measurement is adjusted to be twice thefirst time interval. In the second embodiment the procedure is similarexcept the intervals are adjusted to obtain a counting rate ratio of5.40.

Apparatus for determining thermal neutron decay times according to theinvention includes a variable oscillator for initiating and terminatingthe measurement intervals. The period of this oscillator determines theratio of thermal neutron counting rates in the first and secondintervals. Preferably, the oscillator controls two gates arranged totransmit pulses from a radiation detector during the first and secondintervals and also operates a third gate at a later time to providebackground count information. The background counting rate is subtractedfrom the counting rates during the first and second time intervals and aratio detector computes the ratio of the net counting rates. Adifference circuit responds to any difference between the measuredcounting rate ratio and the value which must result when the first andsecond intervals are one and two decay times, respectively, and adjuststhe oscillator in the proper direction to eliminate any difference.

Further objects and advantages of the invention will be apparent from areading of the following description in conjunction with theaccompanying drawings in which:

FIG. I is a graphical representation illustrating the variation incounting rate versus time after neutron irradiation for three differenttypes of earth formation surrounding a well bore as detected by aradiation detector disposed in the well bore and adapted to respond toindications of thermal neutron intensity;

FIG. 2 is a graphical representation showing the variation in indicateddecay time versus time after neutron irradiation for the three curvesshown in FIG. 1;

FIG. 3 is a graphical representation showing a family of curves havingdifferent scale factors;

FIG. 4 is a graphical representation showing the variation in the ratioof selected areas under the curves of FIG. 3 with changes in the scaleof measurement.

FIG. 5 is a graphical representation of detector counting rate versustime illustrating one embodiment of the method for determining thermalneutron decay time according to the invention;

FIG. 6 is a graphical representation of detector counting rate versustime illustrating another embodiment of the method for determiningthermal neutron decay time; and

FIG. 7 is a schematic block diagram illustrating a representativeapparatus for measuring thermal neutron decay times according to theinvention.

In the graphical representation of FIG. 1, three curves I0, 11 and 12represent, respectively, the logarithm of the counting rate afterbackground subtraction versus time for a detector of thermal neutrons orof neutron capture gamma rays disposed in a well bore following neutronirradiation of formations having short, medium and long decay times orrates of decrease of thermal neutron intensity. The curve 10 representsthe response of a 40 percent porous sandstone formation containingbrackish water having about 250,000 ppm. of salt, while the curve 11indicates the response of [8 percent porous sandstone containing oil andwater, the curve 12 showing the response obtained from zero porositysandstone. For purposes ofcomparison, the curves of FIG. I have beennormalized to ap proximately the same peak counting rate although, inactual practice, this does not generally occur. The variation incounting rate with respect to time may, in each case. be expressed as afunction M(t) and at any time t, the decay time T is defined as WhenM(t) is a decreasing exponential function, T is a constant equal to thetime required for the counting rate to decrease by the factor e.

It will be readily apparent that the decay time T will vary with timewhenever the logarithm of counting rate versus time as represented inFIG. I is not a straight line. In this regard, the curves I4, 15 and 16of FIG. 2 illustrate the variation in "r with time for the typicalcounting rate curves 10, 11 and 12 of FIG. 1. It will be observed fromthese curves that, in each case, the decay time curve has an initialportion 14a, 15a, 16a which increases with time a central portion 14b,15b, 16b in which the decay time is substantially constant and a finalportion 14c, 15c, 16c in which the decay time again increases with time.Because the constant decay time portions 14b, 15b, and 16b, which mostclosely represent the actual decay time of the formation material, donot occur at the same time, it is impossible to select any specific timeinterval after neutron irradiation for measurement of decay times whichwill provide an accurate decay time indication for all types of earthformations. For example, the proper time interval for measuring thedecay time of the material producing the curve 14 is about 200 to 500microseconds after irradiation, whereas that for the curve 15 is about1,000 to 2,500 microseconds after irradiation and that for the curve 16is about [,800 to 4,500 microseconds after irradiation.

In accordance with the invention, therefore, the decay time measurementis made at a time after irradiation which is dependent upon the value ofthe decay time so that the time of measurement coincides with the timeat which the curve provides an accurate decay time indication, i.e.,where the decay time is substantially constant. Stated another way, theamplitude and time scales of the curves 10, I1 and 12 are each changedby factors which cause the curves to coincide over the regions whichcorrespond to the central region 14b, 15b, and 16b, of the correspondingcurves in FIG. 2 the time scale factor for each curve being the decaytime 1' for the curve. Inasmuch as the time scale factor must be r toprovide the desired result, let the counting rate corresponding to aparticular curve be where In this notation the term (i) is used assuperscript and not as an exponent. Since the various functions Mi(t)differ by scale factors in both time and amplitude in the constant 1'region, the appropriate time scale factors 1', and amplitude scalefactors A define for each function Mi(t) a corresponding function A [Ni]N,(x A family of such curves is shown in FIG. 3. In the constant 1'region of interest, however, these functions are approximated by thefunction N(x)=Ae", (4) as indicated by the common portion 13 of thecurves in FIG. 3.

In order to determine the scale factor 'r for any given counting ratecurve, the ratio of the areas under the as ymptotic curve Ae for twoadjoining x intervals I and [1 in the common region 13 is determined.The relative position and widths of the intervals are chosen to optimizethe accuracy of the decay time measurement. For the first embodiment thepositions of the interval boundaries are depicted on FIG. 3 wherein theyare defined to be in a specific proportion to a single parameter X. Theratio of the areas R is therefore a monotonically increasing function ofX. which is shown on FIG. 4 by the curve 13a. Therefore any desired position of the X intervals and in particular the region 13 on FIG. 3, canbe selected by specifying the associated value of Rv As indicated byequation (3), for constant 1' as occurs in the central region of each ofthe thermal neutron decay curves I0, 11 and I2, there will be twocorresponding time intervals I, and II for which the ratio of the areasunder the curves, i.e., the ratio of counting rates during the timeintervals, will be the same as the ratio of the areas I and II under thecurve Ae The correct value of the scale factor for each curve which willcause this equality of the counting rate ratio curve to the asymptoticratio curve is the decay time 1-. The measurement is performed bymaintaining the counting intervals and the interval from the end ofirradiation to the first counting interval in the same fixed proportionto a time parameter t, as the corresponding x intervals bear to theparameter X. The parameter t, is then varied until the ratio of theinterval counting rates is equal to the selected value R0.

The required scale factor or decay time T, is then obtained fromequation (3) by substituting X the value of X, associated with R and forthe value of t. which produces the counting rate ratio equal to R andsolving for 11,-. Since the functions AiNi(X essentially coincide withAe' in the region 13 on FIG. 3.the value of T determined in this mannerwill be unique and will not vary substantially with the particular valueselected for R0 provided that it corresponds to intervals in theasymptotic region 13 on FIG. 3.

Consideration of the shape of the counting rate func tion indicates thatthe most accurate determinations of the scale factor are provided whenthe counting intervals begin at least two decay times after terminationof the neutron irradiation and the second counting interval is longerthan the first. In a particular example illustrated in FIG. 5 whereinneutron irradiation bursts l7 and 18 are provided at nine decay timeintervals as a downhole instrument is moved through the well bore, thecurve 19 represents the thermal neutron count as measured by a detectorin the well bore at subsequent times indicated in terms of the decaytime T. As a practical matter, the decay time usually varies betweenabout 70 microseconds and about l,000 microseconds depending upon thenature of the formation material as indicated by FIG. 2, so that theseparation of nine decay periods from the beginning of one neutron burstto the beginning of the next may be as low as about 630 microseconds forformations having a rapidly decaying thermal neutron concentration andas high as about 9,000 microseconds for formations having slowlydecaying thermal neutron concentration. Moreover, the length of theneutron irradiation periods 17 and 18 is also scaled according to thedecay time of the formation and, in the illustrated example, the neutronirradiation lasts for an interval of two decay times. Where more intenseneutron irradiation is possible, however, the duration of neutronirradiation may be reduced so as to leave more time for measurementbetween irradiation bursts, or closer spacing of bursts.

In the example of FIG. 5, optimum measurement of the decay time isobtained by initiating the first counting rate measurement interval 20two decay times after termination of the irradiation burst l7 and makingthat interval one decay time long. The second counting rate measurementinterval 21 commences immediately after the first and is two decay timeslong while the remaining interval 22 of two decay times before the nextirradiation period 18 is used to provide a measurement of the backgroundcounting rate. After subtraction of the counting rate in the interval 22from that in the interval 21 and one-half of the interval 22 countingrate from that in the interval 20, the ratio of the net counting ratesin the first two intervals is taken.

For an exponential decrease in counting rate as occurs in the centralsection of the curve 19, the ratio of the net counting rates in theintervals 20 and 21 should be 1.99 if the time intervals are actuallyone decay time and two decay times long, respectively. Accordingly, thetime scale of the measuring operation is adjusted until the ratio ofcounting rates is equal to about 1.99 and, when that condition obtains,the duration of the first measuring time interval 20 is equal to thedecay time. This may be indicated either by providing a signalrepresenting the number of microseconds elapsed during the firstcounting interval or, where the time scale is varied by utilization of avariable frequency oscillator, as in the example described hereinafter,by providing an output signal having an inverse relation to thefrequency of the oscillator. The macroscopic capture cross section isinversely proportional to the decay time and may therefore be obtainedby bringing out a signal proportional to the oscillator frequency.

The accuracy of determination of the decay time may be increased, ifdesired, by separating the first and second measurement intervalssomewhat. In the further embodiment of the invention shown in FIG. 6,the first counting rate measuring interval 23 is one decay time long andstarts two decay times after the end of the neutron burst 24 which istwo decay times long as in the previous embodiment. In this case, aninterval of time 25, which is one decay time long, separates the firstcounting interval 23 from the second counting interval 26, the latterbeing two decay times in length. This procedure has been found to reducethe uncertainty in measured decay time to about 60 percent of thatobtained when the example illustrated in FIG. 3 is used when thebackground counting rate is relatively low.

In order to provide a background counting rate measurement which isaffected less by the decaying counting rate caused by the thermal decayprocess, the neutron irradiation bursts in the FIG. 6 example occur atintervals of eight decay times, except that every fourth irradiationburst 27 is suppressed and the background counting rate is measured inan interval 28 six decay times long which starts at the end of thesuppressed neutron burst 27. This background counting rate is subtractedfrom the measurements made during the second interval 26 following eachof the three succeeding irradiation bursts, after which anotherbackground counting rate measurement is made when the next irradiationburst is omitted. Similarly, this background counting rate is divided bytwo for proper normalization before it is subtracted from the countingrate measured during first rate interval 23 following each of the threesucceeding irradiation bursts. For an asymptotic curve, the ratio of theareas in first and second intervals corresponding to the intervals 23and 26 is approximately 5.40. Accordingly, in this case, the time scaleof the measurement is adjusted until the ratio of net counting rates inthe intervals 23 and 26 is 5.40 and, when that condition obtains, theduration of the first interval 23 is equal to the decay time.

in a representative arrangement of apparatus for measuring decay timesaccording to the invention illustrated in FIG. 7, a downhole instrument30 is drawn through a well bore 31 by a multiconductor cable 32. Withinthe instrument 30, a burst neutron source 33, which may be of the typedescribed in the U.S. Pat. No. 2,99l,364 issued Jul. July 4, 196i, toGoodman, for "Well Logging," is positioned to irradiate the formation 34adjacent to the well bore with neutrons. in addition, a radiationdetector 35 is disposed within the instrument 31 in spaced relationabove the source 33 and is positioned to respond in proportion to theconcentration of thermal neutrons in the formation 34. In this regard,the detector 35 may be either a detector of thermal neutrons as, forexample, a scintillation crystal coated with boron trifluoride or it maybe a gamma ray detector adapted to respond to gamma rays resulting fromcapture of thermal neutrons by nuclei of elements in the formation 34.

At the surface of the earth, a variable frequency oscillator 36 producesoutput pulses on a line 37 which are separated by equal time intervalsTn, the time intervals being controlled in duration by a signal on acontrol line 38 from an oscillator control unit 39. A counter 40,receiving time signals from a clock 41, is actuated by the oscillatoroutput signals on the line 37 at intervals To to transmit to a Tindicator 42 a signal indicating the number of microseconds in theintervals TU. When the ratio of counting rates in the two countingintervals is correct, this number is equal to the decay time T of theformation as explained above.

Five factor units 43, 44, 45, 46 and 47 which may, for example, containconventional flip-flop circuits connected in series to the line 37,successively increase the period of the oscillator output signal byfactors of two so that the output signals from these units occur atintervals of 2T", 4T, 8T, 16Tu, and 321}, respectively. In order tocontrol the time intervals in which neutron irradiation and subsequentthermal neutron concentration measurement takes place, a network oflogic gates 48 receives signals from the oscillator 37 and from theunits 43, 44, 45, 46 and 47 and is arranged to provide output signals atthe required times. The logic gate network may be arranged in anyconventional manner to accomplish the desired result as by utilizingappropriate gates and additional flip-flop units.

The typical apparatus illustrated in FIG. 7 is arranged to operate inthe manner described in connection with FIG. 6 irradiating at intervalsof eight decay times with every fourth neutron burst suppressed, thebackground counting rate being measured after the third neutronirradiation. Accordingly, the logic gate network 48 has four outputconductors 49, 50, 51 and 52 providing signals at selected intervalsaccording to the sequence in dicated in FIG. 6. the logic network beingreset at 32Tn to initiate another series of irradiation and measurementcycles. Operation of the pulsed neutron source 33 is controlled bysignals on the line 52 from the unit 48 so as to initiate irradiationfor a time 2T0 at intervals of ST" except when irradiation is omitted atthe time 24Tu.

Signals from the radiation detector 35 representing the thermal neutronconcentration in the formation 34 following neutron irradiation aretransmitted by way L-l a conductor 56 to three gate units 57, 58 and 59.The gate 57. opened at times 4T. 12T,., and Tn. and closed at 5T, 13Ti|and 21TH respectively transmits sig nals on the line 49 signalsrepresenting the counting rate during the first measuring intervaldesignated 23 in FIG. 6 in each ofthe first three 8T.) cyclesofoperation. Similarly, the gate 58, opened at 6T". MT.) and ZZTm andclosed at 8T0. 16TH and 24Ttl by signals on the line 50, transmitssignals representing the counting rate during the second intervaldesignated 26 in FIG. 6 in each of the first three 8T" cycles ofoperation.

After the time when the fourth neutron irradiation burst has beensuppressed at 24TH, the gate 59 is opened at 26T0 and closed at 32TH bysignals on the line 51 so as to transmit the background counting rateduring the interval designated 28 in FIG. 6. It will be understood, ofcourse, that the background counting rate may be measured at any timeafter 24T" and, if necessary to provide a high enough counting rate, itmay be measured during the entire period from 24TH to 32Tu. This signalis transmitted through a flip-flop 60, which divides the count rate bytwo, to a rate counter 61 and directly to a reference counter 62, thecounters 61 and 62 also being connected to receive signals from thegates 57 and 58 respectively. These counters may be conventionalbidirectional or "forward and backward counters, which count in theforward direction when they receive signals from the gates 57 and 58,respectively, and count in the backward direction when they receivesignals from the background gate 59.

The ratio of the differences representing the net thermal neutronconcentration in the intervals 23 and 26 is determined by reading outthe count in the rate counter when the reference counter has reached apredetermined value as detected by a read gate 63. Both the counters arereset to specific initial states after each readout but the net countaccumulated in the rate counter at the time of readout is transferred toa buffer storage unit 64. Since the net count in the reference counteris equal to the same predetermined value at each readout, the numbersstored in the buffer storage unit 64 at each readout are proportional tothe value of the ratio R of net counting rates in the intervals 23 and26. The count in the buffer storage 64 is converted to an analoguevoltage in a digital to analogue converter unit 65, the output of whichis therefore proportional to the aforesaid counting rate ratio R. Acomparing unit 66 compares this measured counting rate ratio with afixed reference ratio R.) which, in the illustrated example, is 5.40 andany difference is amplified by an amplifier 67 and supplied to theoscillator control unit 39. That unit, in turn, changes the frequency ofthe variable oscillator 36 by way of the control line 38 to maintain thedifference between R and R.) as close to zero as possible. In thiscondition, the reading of the oscillator period Tu on the indicator 42is substantially equal to the decay time characterizing the formationbeing irradiated as exemplified in the curves 10, 11 and 12 of FIG. 1.If desired, the decay time may also be indicated by taking thereciprocal of the oscillator frequency as commanded by the oscillatorcontrol line 38. Moreover, either the indicator 42 or an indicator ofthe reciprocal of oscillator frequency, or both, may be arranged torecord the decay time continuously against depth as the instrument 30moves through the well bore. The information processing features of theemdiment of the invention shown in FIG. 6 can be accomplished by anarrangement of known digital, anatron source 33 is pulsed at intervalsunder the control of signals on the line 52. The resulting indicationsof thermal neutron concentration produced by the detector 35 aretransmitted over the conductor 56 to the gates 57, 58 and 59.Subtraction of the background counting rate detected by the gate 59 inproportion to the length of the intervals 23 and 26 provides a netcounting rate for each interval and the ratio taken by the unit 65 aftera selected count is attained in the second interval is compared in theunit 66 to a desired ratio value. Any difference between these valuescauses the oscillator control unit 39 to correct the frequency of thevariable oscillator 36 so that its period is equal to the thermalneutron decay time of the formation.

From the foregoing, it will be apparent that the method and apparatus ofthe present invention provide not only an extremely simple way ofmeasuring thermal neutron decay times which yields an immediate resultbut also one in which the determination is more accurate because thedurations of neutron irradiation bursts and of counting rate detectionintervals and times are varied in accordance with the decay time,thereby permitting detection at the optimum time in each decay curve andallowing higher counting rates in every instance. It will be understood,moreover, that the method and apparatus of the invention may be used formeasuring other neutron characteristic time constants of a material thanthermal neutron decay time as, for example. the neutron slowing downtime.

Although the invention has been described herein with reference tospecific embodiments, many modifications and variations therein willreadily occur to those skilled in the art. Accordingly, all suchvariations and modifications are included within the intended scope ofthe invention as defined by the following claims.

I claim:

1. A method for logging a characteristic of an unknown materialcomprising irradiating the material with neutrons during at least twospaced irradiation intervals, detecting indications of the neutronconcentration in the material during first and second periods,respectively, after commencement of the corresponding irradiationinterval, determining a neutron characteristic of the material from thedetected indications occurring during at least one measurement intervalduring the respective periods, the time of occurrence of at least onemeasurement interval during the second period being variable, andcontrolling automatically the variable time of at least one measurementinterval during the second period in accordance with the neutroncharacteristic determined from the indications detected during the firstperiod in the same logging run.

2. A method according to claim 1 including the steps of detectingindications of the thermal neutron concenetration in the material duringthe first and second periods. determining the thermal neutron decay timeof the material from indications detected during the first period, andcontrolling the time of at least one measurement interval during thesecond period in a manner tending to optimize the measurement of neutronconcentration indications for determination of the thermal neutron decaytime.

3. A method according to claim 3 1 including the steps of detectingindications of the concentration in the material of neutrons havinggreater than thermal energy during the first and second periods,determining the neutron slowing down time from indications detectedduring the first period, and controlling the time of at least onemeasurement interval during the second period so as to optimize themeasurement of neutron concentration indications for determination ofthe neutron slowing down time.

4. A method for logging a characteristic of an unknown materialcomprising irradiating the material with neutrons during at least twospaced irradiation intervals, detecting indications of the neutronconcentration in the material during first and second periods,respectively, after commencement of the corresponding irradiationinterval, measuring the detected indications at least during a firstmeasurement interval occurring at first times during the respectiveperiods and during a second measurement interval occurring at secondtimes during the respective periods, at least one of the secondirradiation interval, the first and second times in the second period,and the first and second measurement intervals during the second periodbeing variable, determining a neutron characteristic of the materialfrom the measurements made during the first and second measurementintervals in the first period, and controlling automatically theduration of at least one of the second irradiation interval, the firstand second times in the second period, and the first and secondmeasurement intervals in the second period, in accordance with theneutron characteristic of the material determined from the measurementsmade during the first period of the same logging run.

5. A method according to claim 4 wherein the duration of the first timein the second period is controlled in accordance with the neutroncharacteristic determined from the measurements made during the firstperiod.

6. A method according to claim 4 wherein the duration of the second timein the second period is con trolled in accordance with the neutroncharacteristic determined from the measurements made during the firstperiod.

7. A method according to claim 4 wherein the duration of the firstmeasurement interval in the second period is controlled in accordancewith the neutron characteristic determined from the measurements madeduring the first period.

8. A method according to claim 4 wherein the duration of the secondmeasurement interval in the second period is controlled in accordancewith the neutron characteristic determined from the measurements madeduring the first period.

9. A method according to claim 4 wherein the duration of the secondirradiation interval in the second period is controlled in accordancewith the neutron characteristic determined from the measurements madeduring the first period.

10. A method according to claim 4 wherein the durations of the first andsecond selected times in the second period are controlled in accordancewith the neu tron characteristic determined from the measurements madeduring the first period.

11. A method according to claim 4 wherein the durations of the first andsecond measurement intervals in the second period are controlled inaccordance with the neutron characteristic determined from themeasurements made during the first period.

12. A method according to claim 4 wherein the neutron characteristic ofthe material is determined by comparing the measurement made during thefirst measurement interval of the first period with the measurement madeduring the second measurement interval of the first period.

13. A method according to claim 4 wherein the first measurement intervalhas a duration approximately equal to a neutron characteristic timeconstant of the material and the second measurement interval has aduration approximately equal to twice the same neutron characteristictime constant of the material,

14. A method according to claim 4 wherein the first and secondmeasurement intervals are substantially contiguous.

15. A method according to claim 4 wherein the first and secondmeasurement intervals are spaced by an interval approximately equal to aneutron characteristic time constant of the material.

16. A method according to claim 4 wherein the first and secondmeasurement intervals are within a time period extending from abouttwice the characteristic time constant of the material after thecorresponding irradiation interval to about six times the neutroncharacteristic time constant of the material after the correspondingirradiation interval.

[7. A method according to claim 4 wherein the duration of theirradiation intervals is approximately equal to twice the neutroncharacteristic time constant of the material.

18. A method for logging a characteristic of an un known materialcomprising irradiating the material with neutrons during at least twospaced irradiation intervals, detecting indications of the neutronconcentration in the material during first and second periods,respectively, after commencement of the corresponding irradiationinterval, measuring the detected indications at least during a firstmeasurement interval occurring at first times during the respectiveperiods and during a second measurement interval occurring at secondtimes during the respective periods, at least one of the secondirradiation interval, the first and second times in the second period,and the first and second measurement intervals during the second periodbeing variable, determining a neutron characteristic of the materialfrom the measurements made during the first and second measurementintervals in the first period, controlling automatically the duration ofat least one of the second irradiation interval, the first and secondtimes in the second period, and the first and second measurementintervals in the second period, in accordance with the neutroncharacteristic of the material determined from the measurements madeduring the first period of the same logging run, measuring detectedindications representative of background radiation during a thirdmeasurement interval occurring at third times during the first andsecond periods, respectively, and subtracting the measurement obtainedduring the third measurement interval from the measurements obtainedduring the first and second measurement intervals in the same loggingrun in determining the neutron characteristic.

19. A method for logging a characteristic of an unknown materialcomprising irradiating the material with neutrons during at least twospaced irradiation intervals, detecting indications of the neutronconcentration in the material during first and second periods,respectively, after commencement of the corresponding irradiationinterval, measuring the detected indications at least during a firstmeasurement interval occurring at first times during the respectiveperiods and during a second measurement interval occurring at secondtimes during the respective periods, at least one of the secondirradiation interval, the first and second times in the second period,and the first and second measurement intervals during the second periodbeing variable, determining a neutron characteristic of the materialfrom the measurements made during the first and second measurementintervals in the first period, controlling automatically the duration ofat least one of the second irradiation interval, the first and secondtimes in the second period, and the first and second measurementintervals in the second period, in accordance with the neutroncharacteristic of the material determined from the measurements madeduring the first period of the same logging run, measuring detectedindications representative of background radiation during a thirdmeasurement interval occurring at third times during the first andsecond periods, respectively, subtracting the measurement obtainedduring the third measurement interval from the measurements obtainedduring the first and second measurement intervals in the same loggingrun in determining the neutron characteristic, and controllingautomatically the third time in the second period in accordance with theneutron characteristic determined from the measurements made during thefirst period of the same logging run.

20. A method for logging a characteristic of an unknown materialcomprising irradiating the material with neutrons during at least twospaced irradiation intervals, measuring the rate of change of neutronconcentration in the material during corresponding first and secondmeasuring intervals occurring after commencement of the respectiveirradiation intervals, the time of occurrence of the second measuringinterval being variable, and controlling automatically the time of thesecond measuring interval in accordance with a neutron characteristic ofthe material determined from a measurement made during the firstmeasuring interval in the same logging run.

21. Apparatus for logging a characteristic of an unknown material basedupon successive detected indications of the neutron concentration in thematerial following successive irradiations of the material with neutronsduring the same logging run comprising conduc tor means for receivingsignals representing the detected indications and synchronizing signalsrelated to the time of initiation of each neutron irradiation, at leastone variable gate means responsive to the synchronizing signals and theneutron concentration indi cation signals for transmitting neutronconcentration indication signals during at least one correspondingmeasurement interval occurring at at least one measurement time afterreceipt of a synchronizing signal, comparing means responsive to theneutron concentration indication signals transmitted by the gate meansto produce a comparison signal related to the neutron concentration inthe material at the measurement time, and control means responsive tothe comparison signal to produce a control signal for controllingautomatically the operation of the gate means, wherein the control meansincludes variable frequency oscillator means responsive to thecomparison signal to produce a control signal having a controlledfrequency.

22. Apparatus for logging a characteristic of an unknown matcrial basedupon successive detected indications of the neutron concentration in thematerial following successive irradiations of the material with neutronsduring the same logging run comprising conductor means for receivingsignals representing the detected indications and synchronizing signalsrelated to the time of initiation of each neutron irradiation, at leastone variable gate means responsive to the synchronizing signals and theneutron concentration indication signals. for transmitting neutronconcentration indication signals during at least one correspondingmeasurement interval occurring at at least one measurement time afterreceipt of a synchronizing signal, comparing means responsive to theneutron concentration indication signals transmitted by the gate meansto produce a comparison signal related to the neutron concentration inthe material at the measurement time, and control means responsive tothe comparison signal to produce a control signal for controllingautomatically the operation of the gate means.

23. Apparatus according to claim 22 including second gate meansresponsive to the synchronizing signals and the neutron concentrationsignals for transmitting neutron concentration indication signals duringa second measurement interval occurring at a second measurement timeafter receipt of a synchronizing signal.

24. Apparatus according to claim 23 wherein the control means includesmeans for producing a control signal for controlling the selectedmeasurement time of at least one gate means.

25. Apparatus according to claim 23 wherein the control means includesmeans for producing a control signal for controlling the measurementinterval of at least one gate means.

26. Apparatus according to claim 23 wherein the control means includesmeans for producing control signals for controlling the selectedmeasurement time and the measurement intervals of both gate means.

27. Apparatus according to claim 22 including output means responsive tothe control means for providing indications of a characteristic of theunknown material.

28. Apparatus for logging a characteristic of an un known materialcomprising neutron source means for irradiating the material withneutrons for an irradiation interval upon receipt of a control signal,at least one of the time and duration of the irradiation interval beingvariable, detector means for producing indications of the neutronconcentration in the material following each irradiation signal duringthe same logging run, at least one gate means for transmitting signalsfrom the detector means during at least one measurement inter val,comparing means responsive to the neutron concentration indicationsignals transmitted by the gate means to produce a comparison signalrelated to the neutron concentration in the material at the measurementtime, and control means responsive to the comparison signal to produce acontrol signal for controlling automatically the operation of theneutron source means.

29. Apparatus according to claim 28 wherein the control means [includeincludes means for producing a control signal which controls theduration of the irradiation interval.

30. Apparatus according to claim 28 wherein the control means includesmeans for producing a control signal which controls the spacing ofsuccessive irradiation intervalsv Lil 31. Apparatus for logging acharacteristic of an unknown material comprising neutron source meansfor irradiating the material with neutrons for an irradiation intervalupon receipt of a control signal, detector means for producingindications of the neutron concentration in the material following eachirradiation signal during the same logging run, at least one variablegate means for transmitting signals from the detector means during atleast one measurement interval at a measurement time, at least one ofthe measurement interval and the measurement time being variable,comparing means responsive to the neutron concentration indicationsignals transmitted by the gate means to produce a comparison signalrelated to the neutron concentration in the material at the measurementtime, and control means responsive to the comparison signal to produce acontrol signal for controlling automatically the operation of thevariable gate means.

32. Apparatus according to claim 3] wherein the control means includesmeans for producing a control signal which controls the duration of themeasurement interval.

33. Apparatus according to claim 31 wherein the control means includesmeans for producing a control signal which controls the selectedmeasurement time.

34. A method for logging a characteristic of material adjacent a wellbore, comprising:

irradiating the material with neutrons during an irradiation interval;measuring detected indications of the concentration of neutrons in thematerial during each of first and second measuring intervals occurringatfirst and second times, respectively, after commencement of theirradiation interval; measuring detected indications of backgroundradiation in the material during a third measuring interval occurring inthe same logging run at a third time after commencement of theirradiation interval, and

combining the measurements made during the first, second and thirdmeasuring intervals to determine a background corrected neutroncharacteristic of the material.

35. A method according to claim 34 wherein the time ofoccurrence of thethird measuring interval is after the termination of the first andsecond measuring intervals.

36. A method according to claim 35 wherein.-

the times ofoccurrence of the first and second measuring intervals areset to provide measurements of gamma rays resulting from capture ofthermal neutrons and indicative of the concentration of thermal neutronsin the material during the first and second measuring intervals.

37. A method according to claim 36 wherein the neutron characteristicdetermined is the thermal neutron decay time ofthe material, and thetime ofoccurrence of at least said third measuring interval is varied asa function of the thermal neutron decay time.

38. A method according to claim 34 wherein the combining of the first,second and third interval measurements includes subtracting a functionof the third interval measurement from the first and second intervalmeasurements to provide background-corrected first and second intervalmeasurements.

39. A method according to claim 38 wherein the combining of the first,second and third interval measurements further includes deriving a ratiofunction of the background-corrected first and second intervalmeasurements to determine the background-corrected neutroncharacteristic.

40. A method according to claim 38 wherein the third intervalmeasurement is subtractedfrom each ofthe first and second intervalmeasurements in proportion to the respective durations oftheflrst andsecond measuring intervals.

41. A method according to claim 34 wherein the step ofcombining thefirst, second and third interval measurements comprises:

determining an indication of the rate of decay of at least a portionofthe neutron population in the materialfrom thefirst and secondinterval measurements; and

correcting the indication of the neutron decay rate in accordance withthe third interval measurement to provide a background-correctedindication of the decay rate. 42. A method according to claim 41 whereinthe neutron decay rate indication is corrected by subtracting a functionof the third interval measurement from the neutron concentrationmeasurements of the first and second measuring intervals.

43. A method for logging a characteristic of material adjacent a wellbore, comprising:

irradiating the material with neutrons during a timed sequence oftime-spaced irradiation intervals;

measuring detected indications of the concentration of neutrons in thematerial during each offirst and second measuring intervals occurring atfirst and second times, respectively, after corresponding irradiationintervals;

measuring detected indications of background radiation in the materialduring a third measuring interval at a third time occurring afterfirstand second measuring intervals following each irradiation interval andprior to the next successive irradiation interval; and

combining the measurements made during the first,

second and third measuring intervals to determine a background correctedneutron cltracteristic of the material.

44. A method according to claim 43 wherein the third measuring intervaloccurs immediately before the next successive irradiation interval.

45. A method according to claim 43 wherein the neutron characteristicdetermined is the thermal neutron decay time of the material. the timingof at least one of said irradiation internal andflrst, second and thirdmeasuring intervals in successive sequences being varied as a functionof decay time determined in prior sequences.

46. A method for logging a characteristic of material adjacent a wellbore, comprising:

irradiating the material with discrete bursts of neutrons in a timedsequence; measuring detected indications of the concentration ofneutrons in the material during each offirst and sec and measuringintervals occurring atfirst and second times, respectively, during thetime period between the commencement of each neutron burst and thecommencement of the next successive neutron but.

suppressing a subsequent neutron burst in the timed sequence;

measuring detected indications of background rudiw tion in the materialduring a third measuring interval occurring during the time period between the predetermined time of commencement of the suppressed neutronburst and the commencement of the next unsuppressed neutron burst; and

combining the measurements made during the first,

second and third measuring intervals to determine a background-correctedneutron characteristic of the material. 47. A method according to claim46 wherein the combining of the first, second and third intervalmeasurements includes subtracting a function of the third intervalmeasurement from the first and second interval measurements to providebackground-corrected first and second interval measurements.

48. A method according to claim 47 wherein the third intervalmeasurement is subtracted from each of the first and second intervalmeasurements in proportion to the respective durations ofthe first andsecond measuring intervals.

49. A method according to claim 46 wherein the second measuring intervalimmediately precedes the time of commencement of suppressed orunsuppressed neutron bursts in the sequence.

50. Apparatus for logging a characteristic of material adjacent a wellbore. comprising:

control means for timing a sequence of intervals during which thematerial is irradiated with neutrons;

means for measuring detected indications of the con centration ofneutrons in the material during each of first and second measuringintervals occurring at first and second times, respectively, aftercommencement ofrespective irradiation intervals;

means for measuring detected indications of background radiation in thematerial during a third measuring interval occurring at a third timebetween successive irradiation intervals; and

means for combining the measurements made during the first, second andthird intervals to determine a background-corrected neutronchracteristic of the material.

51. Apparatus according to claim 50 wherein the meansfor measuringdetected indications of neutron concentration includesfirst and secondgate meansfor transmitting detected indications of the neutronconcentration during saidfirst and second measuring intervals,respectively, and

the means for measuring detected indications of background radiationincludes third gate means for transmitting detected indications ofbackground radiation during said third measuring interval, the thirdgate means being operative to place the time of occurrence of the thirdmeasurirtg interval in sequence following the first and second measuringintervals between successive irradiation intervals. bining meansincludes means for subtracting a function ofthe third intervalmeasurementfrom the first and second interval measurements to providebackgroundcorrected first and second interval measurements.

53. Apparatus according to claim 52 wherein the subtracting means isoperative to subtract the third interval measurementfrom thefirst andsecond interval measuret u't'ttls in proportion to the respectivedurations of the first and second measuring intervals; and

wherein the combining means further includes means for deriving a ratiofunction of the backgroundcorrected first and second intervalmeasurements to determine a background-corrected neutron characteristic.

54. Apparatus according to claim 50 wherein the combining meansincludes:

means responsive to the first and second interval measurements fordetermining an indication of the rate of decay of at least a portion ofthe neutron population in the material; and

means for correcting the indication of the neutron decay rate inaccordance with the third interval measurement to provide abackground-corrected indication of the decay rate.

55. Apparatus according to claim 54 wherein the correcting means isoperative to subtract a function of the third interval measurement fromthe neutron concentration measurements of the first and second measuringintervals.

56. Apparatus according to claim 50 wherein the combining meansdetermines the thermal neutron decay time of the material; and

wherein the timing means varies the occurrence of at least one of themeasuring intervals in successive sequences as a function of the thermalneutron decay time determined in prior sequences.

7. Apparatus according to claim 56 wherein said timing means varies theoccurrence of each of the measuring intervals in successive sequences asa function of the thermal neutron decay time determined in priorsequences.

58. Apparatus according to claim 56 wherein said timing means alsovaries the occurrence of the irradiation intervals in successivesequences as a function of the thermal neutron decay time determined inprior sequences.

59. Apparatus according to claim 50 wherein the timing means controlsthe timed sequence of at least said measuring intervals to place thethird measuring interval after the second measuring interval andimmediately before the next irradiation interval.

60. Apparatus for logging a characteristic ofa materiai adjacent a wellbore, comprising:

means for irradiating the material with discrete bursts of neutrons in apredetermined sequence;

means for measuring detected indications of the concentration ofneutrons in the material during each of first and second measuringintervals occurring at first and second times, respectively, during thetime period between the commencement of each neutron burst and thecommencement of the next successive neutron burst;

means for suppressing a subsequent neutron burst in the timed sequence;

means for measuring detected indications of background radiation in thematerial during a third measuring interval occurring during the timeperiod between the predetermined time of commencement of the suppressedneutron burst and the commencement of the next unsuppressed neutronburst; and

means for combining the measurements made during the first, second andthird measuring intervals to determine a background-corrected neutroncharacteristic of the material.

61 Apparatus according to claim wherein the third interval measuringmeans includes means for commencing and terminating the third measuringinterval in substantial coincidence with the time of commencement of thesuppressed neutron burst and the commencement of the next unsuppressedneutron burst.

62. Apparatus according to claim 60 wherein the third interval measuringmeans includes means for commencing the third measuring interval at thepredetermined time of termination of the suppressed neutron burst andterminating it at the time of commencement of the next unsuppressedneutron burst.

63. Apparatus according to claim 22 including variable gate meansresponsive to the synchronizing signals and the neutron concentrationindication signals for transmitting background indication signals duringa measurement interval occurring at a time after receipt ofasynchronizing signal later than the one measurement time;

said comparing means being responsive to the background indicationsignals to provide background correction for said comparison signal.

64 Apparatus according to claim 3] including variable gate means fortransmitting background indication signals from the detector meansduring a background measurement interval at a time later than saidmeasurement time;

said comparing means being responsive to the background indicationsignals to provide background correction for said comparison signal.

65. Apparatus according to claim 64 wherein the comparing means includesmeans for subtracting a function of the background indication signalsfront the neutron concentration indication signals to provide backgroundcorrection.

66. Apparatus according to claim 65 wherein the subtracting means isoperative to subtract the background indication signals from the neutronconcentration indication signals in proportion to the duration of theneutron concentration measurement interval.

67. A method according to claim 18 wherein the measurement obtainedduring the third measurement interval is subtractedfrom the measurementsobtained during the first and second measurement intervals in proportionto the respective durations of the first and second measurementintervals.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT NO. RE 28,477 DATED July 8, 1975 I E Q) William B. Nelligan It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Col. 1, line 26, "concentrations" should read --c0ncentration; Col. 2,line 13, "materials" should read material-; Col. 1, line 2 "region"should read -regions line 37, "M (t) M r N N (x (2) Ti Ti H 1 shouldread -M (t) M r t N i N (x line 45, after "as" in ert -a-; Mame 5o, "A[Ni]N,(x l should read -A [Ni]N (x Col. line 23, after "for" insert -t;

line 25, "m should read "'-Ti"-; "AiNi(X should read -AiNi (X( Col. 13,line 61, "includes" should not be italicized; Col. 15, line 50,"internal" should read interval-; Col. 18, line 47, after "interval"insert --in which said neutron concentration indication signals weretransmitted-.

Signed and Scaled this sixteenth Day Of March 1976 [SEAL] A nest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner uj'laremsand Trademarks

1. A method for logging a characteristic of an unknown materialcomprising irradiatinG the material with neutrons during at least twospaced irradiation intervals, detecting indications of the neutronconcentration in the material during first and second periods,respectively, after commencement of the corresponding irradiationinterval, determining a neutron characteristic of the material from thedetected indications occurring during at least one measurement intervalduring the respective periods, the time of occurrence of at least onemeasurement interval during the second period being variable, andcontrolling automatically the variable time of at least one measurementinterval during the second period in accordance with the neutroncharacteristic determined from the indications detected during the firstperiod in the same logging run.
 2. A method according to claim 1including the steps of detecting indications of the thermal neutronconcenetration in the material during the first and second periods,determining the thermal neutron decay time of the material fromindications detected during the first period, and controlling the timeof at least one measurement interval during the second period in amanner tending to optimize the measurement of neutron concentrationindications for determination of the thermal neutron decay time.
 3. Amethod according to claim (3) 1 including the steps of detectingindications of the concentration in the material of neutrons havinggreater than thermal energy during the first and second periods,determining the neutron slowing down time from indications detectedduring the first period, and controlling the time of at least onemeasurement interval during the second period so as to optimize themeasurement of neutron concentration indications for determination ofthe neutron slowing down time.
 4. A method for logging a characteristicof an unknown material comprising irradiating the material with neutronsduring at least two spaced irradiation intervals, detecting indicationsof the neutron concentration in the material during first and secondperiods, respectively, after commencement of the correspondingirradiation interval, measuring the detected indications at least duringa first measurement interval occurring at first times during therespective periods and during a second measurement interval occurring atsecond times during the respective periods, at least one of the secondirradiation interval, the first and second times in the second period,and the first and second measurement intervals during the second periodbeing variable, determining a neutron characteristic of the materialfrom the measurements made during the first and second measurementintervals in the first period, and controlling automatically theduration of at least one of the second irradiation interval, the firstand second times in the second period, and the first and secondmeasurement intervals in the second period, in accordance with theneutron characteristic of the material determined from the measurementsmade during the first period of the same logging run.
 5. A methodaccording to claim 4 wherein the duration of the first time in thesecond period is controlled in accordance with the neutroncharacteristic determined from the measurements made during the firstperiod.
 6. A method according to claim 4 wherein the duration of thesecond time in the second period is controlled in accordance with theneutron characteristic determined from the measurements made during thefirst period.
 7. A method according to claim 4 wherein the duration ofthe first measurement interval in the second period is controlled inaccordance with the neutron characteristic determined from themeasurements made during the first period.
 8. A method according toclaim 4 wherein the duration of the second measurement interval in thesecond period is controlled in accordance with the neutroncharacteristic determined from the measurements made during the firstperiod.
 9. A method according to claim 4 wherein the duration of theseconD irradiation interval in the second period is controlled inaccordance with the neutron characteristic determined from themeasurements made during the first period.
 10. A method according toclaim 4 wherein the durations of the first and second selected times inthe second period are controlled in accordance with the neutroncharacteristic determined from the measurements made during the firstperiod.
 11. A method according to claim 4 wherein the durations of thefirst and second measurement intervals in the second period arecontrolled in accordance with the neutron characteristic determined fromthe measurements made during the first period.
 12. A method according toclaim 4 wherein the neutron characteristic of the material is determinedby comparing the measurement made during the first measurement intervalof the first period with the measurement made during the secondmeasurement interval of the first period.
 13. A method according toclaim 4 wherein the first measurement interval has a durationapproximately equal to a neutron characteristic time constant of thematerial and the second measurement interval has a durationapproximately equal to twice the same neutron characteristic timeconstant of the material.
 14. A method according to claim 4 wherein thefirst and second measurement intervals are substantially contiguous. 15.A method according to claim 4 wherein the first and second measurementintervals are spaced by an interval approximately equal to a neutroncharacteristic time constant of the material.
 16. A method according toclaim 4 wherein the first and second measurement intervals are within atime period extending from about twice the characteristic time constantof the material after the corresponding irradiation interval to aboutsix times the neutron characteristic time constant of the material afterthe corresponding irradiation interval.
 17. A method according to claim4 wherein the duration of the irradiation intervals is approximatelyequal to twice the neutron characteristic time constant of the material.18. A method for logging a characteristic of an unknown materialcomprising irradiating the material with neutrons during at least twospaced irradiation intervals, detecting indications of the neutronconcentration in the material during first and second periods,respectively, after commencement of the corresponding irradiationinterval, measuring the detected indications at least during a firstmeasurement interval occurring at first times during the respectiveperiods and during a second measurement interval occurring at secondtimes during the respective periods, at least one of the secondirradiation interval, the first and second times in the second period,and the first and second measurement intervals during the second periodbeing variable, determining a neutron characteristic of the materialfrom the measurements made during the first and second measurementintervals in the first period, controlling automatically the duration ofat least one of the second irradiation interval, the first and secondtimes in the second period, and the first and second measurementintervals in the second period, in accordance with the neutroncharacteristic of the material determined from the measurements madeduring the first period of the same logging run, measuring detectedindications representative of background radiation during a thirdmeasurement interval occurring at third times during the first andsecond periods, respectively, and subtracting the measurement obtainedduring the third measurement interval from the measurements obtainedduring the first and second measurement intervals in the same loggingrun in determining the neutron characteristic.
 19. A method for logginga characteristic of an unknown material comprising irradiating thematerial with neutrons during at least two spaced irradiation intervals,detecting indications of the neutron concentration in the materialduring first and second periods, respectively, afteR commencement of thecorresponding irradiation interval, measuring the detected indicationsat least during a first measurement interval occurring at first timesduring the respective periods and during a second measurement intervaloccurring at second times during the respective periods, at least one ofthe second irradiation interval, the first and second times in thesecond period, and the first and second measurement intervals during thesecond period being variable, determining a neutron characteristic ofthe material from the measurements made during the first and secondmeasurement intervals in the first period, controlling automatically theduration of at least one of the second irradiation interval, the firstand second times in the second period, and the first and secondmeasurement intervals in the second period, in accordance with theneutron characteristic of the material determined from the measurementsmade during the first period of the same logging run, measuring detectedindications representative of background radiation during a thirdmeasurement interval occurring at third times during the first andsecond periods, respectively, subtracting the measurement obtainedduring the third measurement interval from the measurements obtainedduring the first and second measurement intervals in the same loggingrun in determining the neutron characteristic, and controllingautomatically the third time in the second period in accordance with theneutron characteristic determined from the measurements made during thefirst period of the same logging run.
 20. A method for logging acharacteristic of an unknown material comprising irradiating thematerial with neutrons during at least two spaced irradiation intervals,measuring the rate of change of neutron concentration in the materialduring corresponding first and second measuring intervals occurringafter commencement of the respective irradiation intervals, the time ofoccurrence of the second measuring interval being variable, andcontrolling automatically the time of the second measuring interval inaccordance with a neutron characteristic of the material determined froma measurement made during the first measuring interval in the samelogging run.
 21. Apparatus for logging a characteristic of an unknownmaterial based upon successive detected indications of the neutronconcentration in the material following successive irradiations of thematerial with neutrons during the same logging run comprising conductormeans for receiving signals representing the detected indications andsynchronizing signals related to the time of initiation of each neutronirradiation, at least one variable gate means responsive to thesynchronizing signals and the neutron concentration indication signalsfor transmitting neutron concentration indication signals during atleast one corresponding measurement interval occurring at at least onemeasurement time after receipt of a synchronizing signal, comparingmeans responsive to the neutron concentration indication signalstransmitted by the gate means to produce a comparison signal related tothe neutron concentration in the material at the measurement time, andcontrol means responsive to the comparison signal to produce a controlsignal for controlling automatically the operation of the gate means,wherein the control means includes variable frequency oscillator meansresponsive to the comparison signal to produce a control signal having acontrolled frequency.
 22. Apparatus for logging a characteristic of anunknown material based upon successive detected indications of theneutron concentration in the material following successive irradiationsof the material with neutrons during the same logging run comprisingconductor means for receiving signals representing the detectedindications and synchronizing signals related to the time of initiationof each neutron irradiation, at least one variable gate means responsiveto the synchronizing signals and the neutron concentration indicationsignals, for transmitting neutron concentration indication signalsduring at least one corresponding measurement interval occurring at atleast one measurement time after receipt of a synchronizing signal,comparing means responsive to the neutron concentration indicationsignals transmitted by the gate means to produce a comparison signalrelated to the neutron concentration in the material at the measurementtime, and control means responsive to the comparison signal to produce acontrol signal for controlling automatically the operation of the gatemeans.
 23. Apparatus according to claim 22 including second gate meansresponsive to the synchronizing signals and the neutron concentrationsignals for transmitting neutron concentration indication signals duringa second measurement interval occurring at a second measurement timeafter receipt of a synchronizing signal.
 24. Apparatus according toclaim 23 wherein the control means includes means for producing acontrol signal for controlling the selected measurement time of at leastone gate means.
 25. Apparatus according to claim 23 wherein the controlmeans includes means for producing a control signal for controlling themeasurement interval of at least one gate means.
 26. Apparatus accordingto claim 23 wherein the control means includes means for producingcontrol signals for controlling the selected measurement time and themeasurement intervals of both gate means.
 27. Apparatus according toclaim 22 including output means responsive to the control means forproviding indications of a characteristic of the unknown material. 28.Apparatus for logging a characteristic of an unknown material comprisingneutron source means for irradiating the material with neutrons for anirradiation interval upon receipt of a control signal, at least one ofthe time and duration of the irradiation interval being variable,detector means for producing indications of the neutron concentration inthe material following each irradiation signal during the same loggingrun, at least one gate means for transmitting signals from the detectormeans during at least one measurement interval, comparing meansresponsive to the neutron concentration indication signals transmittedby the gate means to produce a comparison signal related to the neutronconcentration in the material at the measurement time, and control meansresponsive to the comparison signal to produce a control signal forcontrolling automatically the operation of the neutron source means. 29.Apparatus according to claim 28 wherein the control means (include)includes means for producing a control signal which controls theduration of the irradiation interval.
 30. Apparatus according to claim28 wherein the control means includes means for producing a controlsignal which controls the spacing of successive irradiation intervals.31. Apparatus for logging a characteristic of an unknown materialcomprising neutron source means for irradiating the material withneutrons for an irradiation interval upon receipt of a control signal,detector means for producing indications of the neutron concentration inthe material following each irradiation signal during the same loggingrun, at least one variable gate means for transmitting signals from thedetector means during at least one measurement interval at a measurementtime, at least one of the measurement interval and the measurement timebeing variable, comparing means responsive to the neutron concentrationindication signals transmitted by the gate means to produce a comparisonsignal related to the neutron concentration in the material at themeasurement time, and control means responsive to the comparison signalto produce a control signal for controlling automatically the operationof the variable gate means.
 32. Apparatus according to claim 31 whereinthe control means includes means for producing a control signal whichcontrols the duration of the measurement interval.
 33. ApparatusaccoRding to claim 31 wherein the control means includes means forproducing a control signal which controls the selected measurement time.34. A method for logging a characteristic of material adjacent a wellbore, comprising: irradiating the material with neutrons during anirradiation interval; measuring detected indications of theconcentration of neutrons in the material during each of first andsecond measuring intervals occurring at first and second times,respectively, after commencement of the irradiation interval; measuringdetected indications of background radiation in the material during athird measuring interval occurring in the same logging run at a thirdtime after commencement of the irradiation interval; and combining themeasurements made during the first, second and third measuring intervalsto determine a background corrected neutron characteristic of thematerial.
 35. A method according to claim 34 wherein the time ofoccurrence of the third measuring interval is after the termination ofthe first and second measuring intervals.
 36. A method according toclaim 35 wherein: the times of occurrence of the first and secondmeasuring intervals are set to provide measurements of gamma raysresulting from capture of thermal neutrons and indicative of theconcentration of thermal neutrons in the material during the first andsecond measuring intervals.
 37. A method according to claim 36 whereinthe neutron characteristic determined is the thermal neutron decay timeof the material, and the time of occurrence of at least said thirdmeasuring interval is varied as a function of the thermal neutron decaytime.
 38. A method according to claim 34 wherein the combining of thefirst, second and third interval measurements includes subtracting afunction of the third interval measurement from the first and secondinterval measurements to provide background-corrected first and secondinterval measurements.
 39. A method according to claim 38 wherein thecombining of the first, second and third interval measurements furtherincludes deriving a ratio function of the background-corrected first andsecond interval measurements to determine the background-correctedneutron characteristic.
 40. A method according to claim 38 wherein thethird interval measurement is subtracted from each of the first andsecond interval measurements in proportion to the respective durationsof the first and second measuring intervals.
 41. A method according toclaim 34 wherein the step of combining the first, second and thirdinterval measurements comprises: determining an indication of the rateof decay of at least a portion of the neutron population in the materialfrom the first and second interval measurements; and correcting theindication of the neutron decay rate in accordance with the thirdinterval measurement to provide a background-corrected indication of thedecay rate.
 42. A method according to claim 41 wherein the neutron decayrate indication is corrected by subtracting a function of the thirdinterval measurement from the neutron concentration measurements of thefirst and second measuring intervals.
 43. A method for logging acharacteristic of material adjacent a well bore, comprising: irradiatingthe material with neutrons during a timed sequence of time-spacedirradiation intervals; measuring detected indications of theconcentration of neutrons in the material during each of first andsecond measuring intervals occurring at first and second times,respectively, after corresponding irradiation intervals; measuringdetected indications of background radiation in the material during athird measuring interval at a third time occurring after first andsecond measuring intervals following each irradiation interval and priorto the next successive irradiation interval; and combining themeasurements made during the first, second and third measuring intervalsto determine a background corrected neutron chracteristic of thematerial.
 44. A method according to claim 43 wherein the third measuringinterval occurs immediately before the next successive irradiationinterval.
 45. A method according to claim 43 wherein the neutroncharacteristic determined is the thermal neutron decay time of thematerial, the timing of at least one of said irradiation internal andfirst, second and third measuring intervals in successive sequencesbeing varied as a function of decay time determined in prior sequences.46. A method for logging a characteristic of material adjacent a wellbore, comprising: irradiating the material with discrete bursts ofneutrons in a timed sequence; measuring detected indications of theconcentration of neutrons in the material during each of first andsecond measuring intervals occurring at first and second times,respectively, during the time period between the commencement of eachneutron burst and the commencement of the next successive neutron burst;suppressing a subsequent neutron burst in the timed sequence; measuringdetected indications of background radiation in the material during athird measuring interval occurring during the time period between thepredetermined time of commencement of the suppressed neutron burst andthe commencement of the next unsuppressed neutron burst; and combiningthe measurements made during the first, second and third measuringintervals to determine a background-corrected neutron characteristic ofthe material.
 47. A method according to claim 46 wherein the combiningof the first, second and third interval measurements includessubtracting a function of the third interval measurement from the firstand second interval measurements to provide background-corrected firstand second interval measurements.
 48. A method according to claim 47wherein the third interval measurement is subtracted from each of thefirst and second interval measurements in proportion to the respectivedurations of the first and second measuring intervals.
 49. A methodaccording to claim 46 wherein the second measuring interval immediatelyprecedes the time of commencement of suppressed or unsuppressed neutronbursts in the sequence.
 50. Apparatus for logging a characteristic ofmaterial adjacent a well bore, comprising: control means for timing asequence of intervals during which the material is irradiated withneutrons; means for measuring detected indications of the concentrationof neutrons in the material during each of rist and second measuringintervals occurring at first and second times, respectively, aftercommencement of respective irradiation intervals; means for measuringdetected indications of background radiation in the material during athird measuring interval occurring at a third time between successiveirradiation intervals; and means for combining the measurements madeduring the first, second and third intervals to determine abackground-corrected neutron chracteristic of the material. 51.Apparatus according to claim 50 wherein the means for measuring detectedindications of neutron concentration includes first and second gatemeans for transmitting detected indications of the neutron concentrationduring said first and second measuring intervals, respectively, andmeans for measuring detected indications of background radiationincludes third gate means for transmitting detected indications ofbackground radiation during said third measuring interval, the thirdgate means being operative to place the time of occurrence of the thirdmeasuring interval in sequence following the first and second measuringintervals between successive irradiation intervals.
 52. Apparatusaccording to claim 50 wherein the combining means includes means forsubtracting a function of the third interval measurement from the firstand second interval measurements to provide background-corrected firstand second interval measurements.
 53. Apparatus according to claim 52wherein the subtracting means is operative to subtract the thirdinterval measurement from the first and second interval measurements inproportion to the respective durations of the first and second measuringintervals; and wherein the combining means further includes means forderiving a ratio function of the background-corrected first and secondinterval measurements to determine a background-corrected neutroncharacteristic.
 54. Apparatus according to claim 50 wherein thecombining means includes: means responsive to the first and secondinterval measurements for determining an indication of the rate of decayof at least a portion of the neutron population in the material; andmeans for correcting the indication of the neutron decay rate inaccordance with the third interval measurement to provide abackground-corrected indication of the decay rate.
 55. Apparatusaccording to claim 54 wherein the correcting means is operative tosubtract a function of the third interval measurement from the neutronconcentration measurements of the first and second measuring intervals.56. Apparatus according to claim 50 wherein the combining meansdetermines the thermal neutron decay time of the material; and whereinthe timing means varies the occurrence of at least one of the measuringintervals in successive sequences as a function of the thermal neutrondecay time determined in prior sequences.
 57. Apparatus according toclaim 56 wherein said timing means varies the occurrence of each of themeasuring intervals in successive sequences as a function of the thermalneutron decay time determined in prior sequences.
 58. Apparatusaccording to claim 56 wherein said timing means also varies theoccurrence of the irradiation intervals in successive sequences as afunction of the thermal neutron decay time determined in priorsequences.
 59. Apparatus according to claim 50 wherein the timing meanscontrols the timed sequence of at least said measuring intervals toplace the third measuring interval after the second measuring intervaland immediately before the next irradiation interval.
 60. Apparatus forlogging a characteristic of a material adjacent a well bore, comprising:means for irradiating the material with discrete bursts of neutrons in apredetermined sequence; means for measuring detected indications of theconcentration of neutrons in the material during each of first andsecond measuring intervals occurring at first and second times,respectively, during the time period between the commencement of eachneutron burst and the commencement of the next successive neutron burst;means for suppressing a subsequent neutron burst in the timed sequence;means for measuring detected indications of background radiation in thematerial during a third measuring interval occurring during the timeperiod between the predetermined time of commencement of the suppressedneutron burst and the commencement of the next unsuppressed neutronburst; and means for combining the measurements made during the first,second and third measuring intervals to determine a background-correctedneutron characteristic of the material.
 61. Apparatus according to claim60 wherein the third interval measuring means includes means forcommencing and terminating the third measuring interval in substantialcoincidence with the time of commencement of the suppressed neutronburst and the commencement of the next unsuppressed neutron burst. 62.Apparatus according to claim 60 wherein the third interval measuringmeans includes means for commencing the third measuring interval at thepredetermined time of termination of the suppressed neutron burst andterminating it at the time of commencement of the next unsuppressedneutron burst.
 63. Apparatus according to claim 22 including variablegate means responsive to the synchronizing signals and the neutronconcentration indication signals for transmitting background indicationsignals during a measurement interval occurring at a time after receiptof a synchronizing signal later than the one measurement time; saidcomparing means being responsive to the background indication signals toprovide background correction for said comparison signal.
 64. Apparatusaccording to claim 31 including variable gate means for transmittingbackground indication signals from the detector means during abackground measurement interval at a time later than said measurementtime; said comparing means being responsive to the background indicationsignals to provide background correction for said comparison signal. 65.Apparatus according to claim 64 wherein the comparing means includesmeans for subtracting a function of the background indication signalsfrom the neutron concentration indication signals to provide backgroundcorrection.
 66. Apparatus according to claim 65 wherein the subtractingmeans is operative to subtract the background indication signals fromthe neutron concentration indication signals in proportion to theduration of the neutron concentration measurement interval.
 67. A methodaccording to claim 18 wherein the measurement obtained during the thirdmeasurement interval is subtracted from the measurements obtained duringthe first and second measurement intervals in proportion to therespective durations of the first and second measurement intervals.